A major source of impairment in all-optical networks is the amplified spontaneous emission (ASE) noise arising from optical amplifiers, usually quantified by the optical signal-to-noise-ratio (OSNR). In static point-to-point Wavelength Division Multiplexing (WDM) networks, the OSNR may be estimated by linearly interpolating out-of-band ASE noise levels (See, e.g., H. Suzuki, N. Takachio, “Optical Signal Quality Monitor Built into WDM Linear Repeaters Using Semiconductor Arrayed Waveguide Grating Filter Monolithically Integrated with Eight Photodiodes,” Electron. Lett., vol. 35, no. 10, pp. 836–837, 1999).
Unfortunately, the OSNR cannot be measured accurately using such spectral methods in transparent reconfigurable dense WDM networks where each channel may traverse different routes, add-drop filters, and optical amplifiers. (See, e.g., D. C. Kilper, S. Chandrasekhar, L. Buhl, A. Agarwal, D. Maywar, “Spectral Monitoring of OSNR in High-Speed Networks,” Proc. ECOC'2002, paper 7.4.4, 2002). To further compound the problem, there is little spectrum available for monitoring between channels.
In-band OSNR monitoring methods that measure noise power within individual channels have to be used for all-optical dense WDM networks. Several methods for in-band OSNR monitoring have been proposed. One approach is to use optical polarization information, such as the degree of polarization (DOP) method and polarization nulling method (See, e.g., M. Petersson, H. Sunnerud, B-E Olsson, M. Karlsson, “Multi-Channel OSNR Monitoring for WDM Networks,” Proc. ECOC'2002, paper 1.1.6, 2002; J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR Monitoring Technique Using Polarization-Nulling Method,” IEEE Photon. Technol. Lett., vol. 13, pp. 88–90, January 2001). Another approach measures the beating term between signal and amplified spontaneous emission (ASE) noise (See, e.g., S. K. Shin, K. J. Park, and Y. C. Chung, “A Novel Optical Signal-to-Noise Ratio Monitoring Technique for WDIM Networks”, Proc. OFC'2000, paper WK6, 2000; C. Dorrer and X. Liu, “Noise Monitoring of Optical Signals Using RF Spectrum Analysis and Its Application to Phase-Shift-Keyed Signals”, IEEE Photon. Technol. Lett., vol. 16, pp. 1781–1783, 2004; and W. Chen, R. S. Tucker, X. Yi, W. Shieh, and J. S. Evans, “Optical Signal-to-Noise Ratio Monitoring Using Uncorrelated Beat Noise”, IEEE Photon. Technol. Lett., vol. 17, pp. 1781–1783, 2005).
Polarization-assisted OSNR monitoring techniques such as these capture and analyze the in-band ASE noise, and they are relatively simple to implement. Unfortunately however, polarization mode dispersion (PMD) and inter-channel cross-phase modulation (XPM) induced nonlinear polarization scattering in WDM systems introduce large errors in these techniques (See, e.g., C. Xie, L. Möller, D. C. Kilper, and L. F. Mollenauer, “Impact of Cross-Phase Modulation Induced Polarization Scattering on Optical PMD Compensation in WDM Systems”, Opt. Lett., vol. 28, no. 23, pp. 2303–2305, 2003; C. Xie and D. C. Kilper, “Influences of Polarization Scattering on Polarization-Assisted OSNR Monitoring in Dense WDM Systems with NZ-DSF and Raman Amplification,” Proc. OFC'2005, paper JWA40, 2005).
Supplemental techniques which use narrow bandwidth optical filters and additional measurements have been employed to reduce errors caused by PMD and the nonlinear polarization scattering effects with limited success. (See, e.g., J. H. Lee, and Y. C. Chung, “An Improved OSNR Monitoring Technique Based on Polarization-Nulling Method,” Proc. OFC'2001, paper TuP6, 2001; M-H. Cheung, L-K. Chen, and C-K. Chan, “A PMD-insensitive OSNR monitoring scheme base on polarization-nulling with off-center narrowband filtering,” Proc. OFC'2004, paper FF2; and M. Skold, B-E. Olsson, H. Sunnerud, and M. Karlsson, “PMD insensitive DOP-based OSNR Monitoring by spectral SOP Measureents,” Proc. OFC'2005, paper OThH3, 2005.)
The prior art OSNR monitoring techniques based upon signal-ASE beating term measurement—while typically polarization independent—impose special signal requirements, such as a particular bit length, special modulation format, or a symmetrical spectrum. Therefore such techniques are simply not suitable for application in real networks.